This invention relates generally to the cryogenic rectification of feed air and, more particularly, to the cryogenic rectification of feed air to produce high purity nitrogen and even ultra high purity nitrogen.
High and ultra high purity nitrogen is used extensively in the manufacture of high value components such as semiconductors where freedom from contamination by oxygen is critical to the manufacturing process. High purity nitrogen is generally produced in large quantities by the cryogenic rectification of feed air using a single column plant or a double column plant. The production of high purity nitrogen is energy intensive and any system which can produce high purity nitrogen with lower power requirements than heretofore available systems would be highly desirable.
Accordingly it is an object of this invention to provide a system for producing high and ultra high purity nitrogen by the cryogenic rectification of feed air which has lower power requirements than do heretofore available comparable conventional systems.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for producing high purity nitrogen comprising:
(A) cooling feed air, passing cooled feed air into a first column, and producing by cryogenic rectification within the first column first high purity nitrogen fluid and first oxygen-enriched fluid;
(B) passing at least a portion of the first oxygen-enriched fluid into a second column and producing by cryogenic rectification within the second column second high purity nitrogen fluid and second oxygen-enriched fluid;
(C) warming second oxygen-enriched fluid to produce oxygen-enriched vapor, and turboexpanding the oxygen-enriched vapor to generate refrigeration;
(D) employing refrigeration from the oxygen-enriched vapor to cool the feed air; and
(E) recovering a portion of the first high purity nitrogen fluid as product high purity nitrogen.
Another aspect of the invention is:
Apparatus for producing high purity nitrogen comprising:
(A) a main heat exchanger, a first column, and means for passing feed air to the main heat exchanger and from the main heat exchanger to the first column;
(B) a second column having a top condenser, and means for passing fluid from the lower portion of the first column into the second column;
(C) means for passing fluid from the lower portion of the second column into the second column top condenser;
(D) a turboexpander, means for passing fluid from the second column top condenser to the turboexpander, and means for passing fluid from the turboexpander to the main heat exchanger; and
(E) means for recovering high purity nitrogen from the upper portion of the first column.
As used herein the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen and nitrogen, such as ambient air.
As used herein the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. 
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein the term xe2x80x9ctop condenserxe2x80x9d means a heat exchange device that generates column downflow liquid from column vapor.
As used herein the terms xe2x80x9cturboexpansionxe2x80x9d and xe2x80x9cturboexpanderxe2x80x9d mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration.
As used herein the term xe2x80x9csubcoolingxe2x80x9d means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure.
As used herein the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d mean those sections of a column respectively above and below the mid point of the column.
As used herein the term xe2x80x9chigh purity nitrogenxe2x80x9d means a fluid having a nitrogen concentration of at least 99 mole percent, preferably at least 99.9 mole percent, most preferably at least 99.999 mole percent. A particularly desirable form of high purity nitrogen is ultra high purity nitrogen which is a fluid having a nitrogen concentration of at least 99.999999 mole percent.